This invention relates to a noise control system for suppressing or eliminating noise in the audio output of radio receivers. More specifically, this invention relates to an improved noise detection and control system for use in FM stereo radio receivers installed in vehicles, and particularly for use in suppressing or eliminating noise such as multipath distortion occurring in areas where the broadcast signal strength is relatively high.
In radio reception, the signal to noise ratio of the audio output comprises an extremely important parameter defining the quality of that output. While the specific quality of the audio output is highly subjective according to the taste of each individual listener and is subject to other parameters such as frequency response, stereo channel separation, and the like, it is fundamental that the signal/noise ratio must be greater than some arbitrary threshold to provide an audio output for satisfactory listening. For most applications and for most listeners, it is highly desirable to provide an audio output having a signal/noise ratio which is sufficiently high such that background noise or interference in the audio output is no more than barely discernible to the average listener.
As a general rule, the signal/noise ratio of the audio output varies inversely as a function of the strength of the broadcast radio signal. Thus, when the broadcast signal strength is relatively strong, typically when the radio receiver is positioned physically near the transmission tower such as in a metropolitan area, the signal/noise ratio tends to be relatively high resulting in a satisfactory audio output. However, as the physical location of the radio receiver is moved away from the transmission tower, such as when the receiver is positioned in a remote or outlying area, noise level increases to result in a decreasing signal/noise ratio and a deteriorating audio output.
In FM stereo radio reception, a broadcast FM stereo signal is decoded or demodulated to produce right and left channel audio outputs. In the United States, decoding of the broadcast stereo signal is normally achieved by a so-called multiplexing circuit, typically in the form of a solid state chip. However, multiplexing of the broadcast signal substantially reduces the signal/noise ratio in the audio output. While this decrease in signal/noise ratio may not be normally discernible or critical in a high signal strength area, it is readily discernible in the audio output in a low signal strength area.
In the prior art, noise suppression systems have been proposed for controlling noise in an FM stereo radio receiver by switching operation of the receiver from stereo to monaural operation in response to the strength of the broadcast signal. More specifically, when the stereo receiver is switched to a monaural state, the signal/noise ratio loss incurred by multiplexing is eliminated to increase significantly the signal/noise ratio by as much as about 16 dB or more, and thereby suppress system noise. In some radios, this switching is achieved by a manual stereo/monaural selector button to enable the operator to change the mode of radio receiver operation according to his individual desires for stereo operation and his tolerance for noise. Other prior art systems have been proposed which automatically adjust or modulate the degree of stereo channel separation as an arbitrary function of broadcast radio signal strength without requiring manual operator intervention. In addition, some of these automatic systems have been combined with frequency response adjustment circuitry for reducing high frequency response of the radio receiver upon decreases in signal strength to filter at least some of the noise from the audio output, since a substantial portion of the noise tends to occur at the higher frequency levels. Thus, when the receiver is installed in a vehicle moving away from the transmission tower, the noise suppression system reduces the stereo channel separation and/or the high frequency response of the audio output as broadcast signal strength decreases to maintain a satisfactorily high signal/noise ratio of the audio output.
A major problem remains, however, in FM stereo radio receivers operated in high strength signal areas, and particularly when the receiver is installed in a vehicle moving rapidly through a high strength signal area. More specifically, a phenomena referred to as "multipath distortion" tends to occur in high strength signal areas wherein a variety of radio signals having substantially the same frequency arrive at the radio receiver at the same time, but with differing magnitudes and/or phase relationships when compared with the primary radio signal. These various signals can be created, for example, by means of the primary signal bouncing off or being reflected by buildings or other interferring structures in the radio field area. Alternately, these signals can arise by independent generation of unrelated signals as a result of a virtually infinite variety of electronic signals present in a metropolitan area. In any event, the effect of these signals is to superimpose a high level of noise upon the audio output even in a high strength signal area, and this undersirable effect is significantly enhanced when the receiver is installed in a vehicle and moved rapidly through the radio transmission field. Prior art noise suppression systems, however, have not provided any means for controlling or suppressing this noise in a high signal strength area.
The present invention comprises an improvement over prior noise suppression systems in FM stereo radio receivers by providing means for controlling the signal/noise ratio of the audio output in response to the presence of relatively high noise levels and substantially independent of the broadcast signal strength.